In recent years, accidents in secondary batteries have occurred. Particularly, ignition and fuming accidents in lithium-ion secondary batteries have been rapidly increased.
In lithium-ion secondary batteries, metal oxides which are easily destabilized in high-temperature conditions are used for positive electrode materials, and electrolytic solutions contain organic solvents. Therefore, when such lithium-ion secondary batteries are in high-temperature conditions, ignition and fuming accidents easily occur. Further, as a cause for which lithium-ion secondary batteries are in high-temperature conditions, there have been mentioned internal short-circuits due to metal particles precipitated in these batteries during charging. Further, there has been the following problem: intrusion of foreign substances during manufacturing of batteries. There have been reports of accidents of short-circuits caused by such foreign substances during use of batteries.
In order to prevent such ignition and fuming accidents, Japanese Industrial Standards (JIS) define safety tests for lithium-ion secondary batteries (for example, JIS C8714 and JIS C 8715-2). As such safety tests according to Japanese Industrial Standards, there are mentioned various tests such as a crushing test, an external short-circuit test, and a forced internal short-circuit test.
Such a forced internal short-circuit test is defined as quickly disassembling a sealing port portion in a charged single battery, taking out an electrode body from a case, disposing a nickel small piece with a determined shape as a test metal piece between an electrode and a separator in an outermost portion, and thereafter, putting the electrode body back into the case, pressing a portion where the test metal piece is disposed, to induce a short-circuit between a positive electrode and a negative electrode, and determining whether or not ignition or fuming occurs.
In large-sized batteries, when a battery has a case made of a rigid material, such as a metal case, the battery has a large electric capacity, and if the case is disassembled and a power generating element is taken out in a charged condition, danger is brought. Therefore, it has been impractical that such a battery is disassembled and the power generating element is taken out. Therefore, in order to easily disassemble the battery even in a charged condition, a forced internal short-circuit test has been conducted by preparing a battery having a pouch made of a laminate film as an alternative battery case.